A polypropylene resin is now increasingly utilized in various fields because of excellent mechanical strengths, heat resistance, chemical resistance, machinability, cost balance and recyclability thereof. Foamed moldings of a base resin including a polypropylene resin (hereinafter referred to simply as “PP moldings” or “polypropylene resin moldings”), which retain the above excellent properties and which have excellent additional characteristics such as cushioning property and heat insulating properties, are thus utilized for various applications as packaging materials, construction materials, heat insulation materials, etc. In particular, PP moldings obtained by heating expanded beads of a base resin including a polypropylene resin (hereinafter referred to as “expanded PP beads” or “expanded polypropylene resin beads”) in a mold are now used as bumper cores and door pats of automobiles because of their good shock absorbing properties and moldability.
Thus, a need for light weight and high rigidity PP moldings is increasing in this field. In one structure of such PP moldings, a dual density molding is known which has a relatively high density section and a relatively low density section. Because of the presence of the low density section, the dual density molding has a reduced weight as a whole as compared with a structure in which no such a low density section is present and is advantageously utilized as a high functional bumper core in which offset collision and pedestrian protection are taken into account. One typical dual-density molding has a center low density section which is sandwiched between a pair of high density sections. In the production of such a dual-density molding, high density expanded PP beads and low density expanded PP beads are filled in predetermined spaces of a mold cavity and are heated to fuse-bond the expanded PP beads into a unitary structure, as disclosed in U.S. Pat. No. 5,164,257, Japanese Laid-Open Patent Publications No. H11-334501 and 2001-150471 and Japanese Utility Model Examined Publication S62-22352. The thus obtained foamed molding is then cooled and taken out of the mold.
Such a dual density PP molding is, however, apt to expand to a size greater than the mold cavity, when the molding is not sufficiently cooled after the fuse-bonding of expanded PP beads has been completed. The PP molding is also apt to shrink to a size smaller than the mold cavity, when the molding is excessively cooled after the fuse-bonding of expanded PP beads has been completed. Thus, depending upon the degree of cooling, the dual density PP molding expands or shrinks. In this case, since a relatively low density section is more quickly cooled than a relatively high density section, the low density section is more likely to shrink, when the cooling of the molding is carried out evenly. Since expansion is less desirable than shrinkage, the cooling is generally carried out while preventing expansion of the high density section. Thus, the shrinkage of the low density section is generally unavoidable in the case of production the dual density PP molding unless specifically controlled cooling conditions are adopted.